My little guy is so thin

I have been trying to fatten him up but he just doesn't seem to gain weight. I spent all week feeding him cheese and little snacks(along with regular food)and he didn't gain an ounce. He is happy and healthy for the most part but I really think he is too thin.
Today when I was holding him while watching TV, I really was looking at him hard and even though I knew he was little , kept thinking how his legs were no larger then a No. 2 pencil. I totally started freaking out after this thinking how really fragile he was . Of course i knew he was fragile from the beginning but after comparing his legs to pencils I started panicking , thinking how easily a pencil can snap.
Anywho.... any suggestions for fattening up a skinny little boy in a healthy manner.

I'm sure the added cheese and pork bites taste good but I can't imagine it's too healthy.